1. Field of the Invention
The present invention relates to an optical disk and an optical disk reproducing apparatus.
2. Description of the Related Art
FIG. 1 shows a part of an optical disk 12. The optical disk 12 includes a substrate 13 having an information recording surface 15 and containing polycarbonate resin, and a protection layer 14 covering the information recording surface 15. A plurality of information pits 11, which have lengths modulated by recording information are recorded on the information recording surface 15. During an information reproducing operation, a laser beam is projected onto one of the information pits 11 so that a laser beam spot LS having a predetermined diameter is formed on the information recording surface 15. A diffracted and reflected light is detected by a photodetector formed with, for example, a photodiode. The photodetector converts the received light into an electric signal. Information, such as audio information and video information, is extracted from the electric signal by means of a signal conversion process, which is the reverse of a signal conversion process for information recording. Examples of optical disks as described above are compact disks (CD) and laser video disks (LVD).
As shown in FIG. 2, an information recording density of optical disks Greatly depends on the value of a track pitch P and the diameter of the laser beam spot LS formed on the information pit 11. The track pitch P is the distance between the center lines of adjacent pit strings, that is, the distance between the tracks. Hence, in order to increase the information recording density, it may be effective to reduce the track pitch P.
In the cases as shown in FIGS. 1 and 2, there is no problem of crosstalk between adjacent tracks. However, according to the study of the present inventor, as shown in FIG. 3, if the track pitch is reduced to be a value P1 smaller than the value P shown in FIG.2, for example, P1=P/2, information pits 11A on two adjacent tracks are simultaneously included within the laser beam spot LS. In this case, a large quantity of crosstalk occurs between the adjacent tracks, and the optical disk shown in FIG. 3 is not suitable for practical use.
In order to increase the information recording density, it may be effective to diminish the size of the laser beam spot LS. As shown in FIG. 4, a minimum beam diameter w obtained by focusing, by means of an objective lens 16, a laser beam having a wavelength .lambda. at a position corresponding to a focusing distance f of the objective lens 16 is expressed as follows: EQU w=1.22.times.(.lambda./NA) (1)
where NA denotes a numerical aperture of the objective lens 16. The numerical aperture NA is defined as follows: EQU NA=n.times.sin.THETA. (2)
where n denotes the refractive index of the objective lens 16, and .THETA. is the angle of emergence from the objective lens 16. Hence, in order to reduce the diameter w of the laser beam spot LS, it is necessary to reduce either the wavelength .lambda. or the numerical aperture NA. The wavelength of a semiconductor laser used for optical disks is approximately equal to 0.780 .mu.m (the .mu.m is equal to 1.times.10.sup.-6 m). The numerical aperture NA for optical disks is approximately equal to 0.45. Hence, the diameter w of the laser beam spot LS is approximated as follows: EQU w.sub.min =1.22.times.(0.780/0.45)=2.1 (.mu.m) (3)
As a result, a minimum track pitch which does not result in crosstalk when the laser beam spot LS is projected onto a pit string is approximately equal to 1.6 .mu.m. Thus, optical disks employ a minimum track pitch of 1.6 .mu.m.